Process of sulfur addition and double rolling treatment to obtain predetermined particle size distribution



July 21, 1964 G. B. FINKE ETAL PROCESS OF SULFUR ADDITION AND DOUBLE ROLLING TREATMENT TO OBTAIN PREDETERMINED Filed June 5, 1962 Final Hol Rolling Temp.= I650 Degree Of Reduclion ll Quench: Boiling Water Roi l i ng Speed 10.5 Meier/Second FIG. 3.

Sulphur=.0l4%

Finul Hot RollingTemp, l6l0 Degree Of Reduclion= 26% Quench: Water, Room Temp- Rolling Speed l.0 Meter/Second PARTICLE SIZE DISTRIBUTION 2 Sheets-Sheet 1 FIG. 2.

Sulphur=.Ol5%

Final Hor Rolling Temp; I650 Degree Of Reduction: 12% Quench: Wurer Ar 200E Rolling Speed:O.5 Meier/Second Sulphur:,Ol2% Final Ho'r Rolling Temp I570 Degree Of Reducfion:

Quench: Circulating W0ler-50 F. Rolling Speed:

LO Meter/Second INVENTORS Guenfer' B. Finke Friedrich W Ackermonn ATTORNEYS July 21, 1964 FINKE ETAL 3,141,760 PROCESS OF SULFUR ADDITION AND DOUBLE ROLLING TREATMENT TO OBTAIN PREDETERMINED PARTICLE SIZE DISTRIBUTION 2 Sheets-Sheet 2 Filed Jung 5, 1962 8 5 m 8: E :5 3 m 0 Om O o o 8 m r 091v un 18d sugmg 4 JBQUJDN INVENTORS Guenfer B. Finke Friedrich W. Ackermann ATTORNE Y5 United States Patent Office 3,141,766 Patented July 21, 1964 3,141,760 PROCESS OF SULFUR ADDITION AND DOUBLE ROLLING TREATMENT T t) OBTAlN PREDETER- MINED PARTlCLE SIZE DISTRIBUTION Guenter B. Finke and Friedrich W. Ackermann, Cherry Hill, N.J., assignors to Magnetic Metals Company, Camden, NJ.

Filed June 5, 1962, Ser. No. 200,149 6 Claims. (Cl. 75-.5)

This invention relates to a method for treating metals so as to obtain a highly embrittled product with a predetermined grain size, and more particularly pertains to a process for producing powdered metals having a predetermined particle size distribution.

It has long been recognized that the mean particle size of the magnetic alloy used in magnetic powder core has pronounced effect upon its electrical characteristics. In general, larger particles, i.e., those in the range of 40-80 microns, are used when high permeability is desired, but much smaller particles, i.e., in the range of 10-20 microns, must be used for high frequency applications to maintain electrical losses at an acceptable figure.

Moreover, it is ordinarily desirable that the density of the magnetic material be as high as possible in order that the core will have the desired electrical characteristics. However, the extent to which physical pressure can be applied in an effort to secure high density is severely limited by the necessity of not rupturing the electrical in sulation between adjacent particles, which is initially effected by the application of a chemical coating to the particles.

It has long been recognized that greater density can be obtained by mixing particles of different sizes as opposed to having all particles substantially the same size. Of course, in making up a blend of different sized powders in order to obtain a mix of high density, consideration must still be given to the electrical characteristics which are desired. In this connection, considerable experimental work has been done to determine the optimum blends which will give high density and yet fulfill the specifications as to permeability and electrical losses.

From the foregoing description, it becomes apparent that a manufacturer of magnetic cores must have on hand magnetic alloy powders having respectively diflerent particle sizes so that the desired proportions of the different sized powders may be selected in accordance with the predetermined specifications. As will be shown, the particle size of the magnetic alloy powders of any heat is dependent upon a considerable number of factors. Because of this, it has heretofore been virtually impossible to control the process of manufacture of the magnetic powders in such a way that a predetermined particle size would be achieved. In the endeavor to overcome this problem, it has become the accustomed practice to sift the magnetic powder through various screens having respectively different mesh sizes so as to segregate the particles according to size and thereby accumulate stocks of each of the different nominal particle sizes. This has by no means constituted a satisfactory solution to the problem since the absence of close control over particle size has meant that, in order to produce an ample quantity of powder of a given particle size, there has necessaritly been uneconomical overproduction of powder having different sizes of particles. For example, if it is desired that a quantity of Permalloy powder be produced with a particle size in the range of 10-20 microns, this has ordinarily required that unwanted quantities of Permally powders in the other sizes have also had to be produced before obtaining the required amount of the desired particle size. By reason of the method of the present invention whereby the particle size can closely be controlled, it is readily possible to tailor the process according to the need so that if powder of a given particle size is required, the next batch can be so treated that the majority of the particles resulting therefrom will be of the desired size.

It is, accordingly, an object of this invention to provide a method for the manufacture of metal powders wherein the resulting particle size can be closely controlled.

In describing the invention, reference will be made to the accompanying photomicrograph which illustrates in FIGURES 1 through 4 thereof various crystalline struc tures having respectively different grain sizes which were obtained by control of the various parameters of the process of this invention, and also to FIGURE 5 which graphically represents the relationship between grain size and degree of reduction.

It has long been recognized that there is a strong tendency for an embrittled metal to fracture along its crystalline boundaries when pulverized. Also, it has been known that embrittlement occurs when there is a substantially uninterrupted layer of non-malleable sulphides on the grain boundaries. Accordingly, the process of this invention resolves to one of quenching and/or cooling the recrystallizing (magnetic) alloy at a time when (a) the grain size corresponds closely to the desired particle size and (b) each grain is substantially covered by a layer of non-malleable sulphides.

The present invention is primarily concerned with the manufacture of Permalloy powders, Permalloy consisting primarily of nickel, 2% molybdenum, and 18% iron. However, this invention is also applicable to the manufacture of metallic powders other than Permalloy and particularly those including as constituents cobalt, iron and nickel which have a speed of recrystallization which is greater over some particular temperature range, than the diffusion speed of sulphur back into the lattice structure.

In practicing the process of this invention, it is preferable to start with Permalloy which includes a minimum amount of magnesium, calcium, or manganese. The reason for this is that these metals form sulphides or oxides which become finely dispersed throughout the grain of the metal but have only slight detrimental effect on its malleability. In other words, these elements have a tendency to capture the sulphur appearing in the alloy so that there is a decrease in the nickel and iron sulphides which are formed, which latter sulphides are highly desirable as previously indicated since their presence on the grain boundaries contributes greatly to the embrittling process. Therefore, when the starting material is a Permalloy, alloy includes manganese, calcium or magnesium, an increased amount of sulphur must be added to compensate for that which has been captured by these ingredients in order that there will be sufiicient free sulphur left to combine with the nickel and iron and provide an adequate antity of the sulphides of these metals. Normally, the desired amount of free sulphur varies in the range of .005% to 020%.

A description will now be given as to the preferred mode of carrying out the process of this invention. Although described in detail with the giving of specific temperature ranges and with specific recitation as to the thickness to which the strip is to be rolled and the like, it should be understood that the scope of the invention is not intended to be limited to the specific details of this preferred mode.

Once the required amount of sulphur has been added to the heat, the metal is cast, after which it can be forged or hot-rolled. Hot-rolling is carried out at temperatures above 2,000 F. since, at such temperatures, the sulphur is in solution and the diffusion rate of the sulphur back into the grains of the alloy is high enough to prevent any agglomeration of the sulphides and oxides on the grain boundaries. Because of this, the material is in a malleable state and can be readily handled throughout the rolling process.

The metal is preferably cast in the form of a strip having a two inch thickness. If no forging is contemplated, it is first subjected to a light rolling. Ordinarily, this first rolling is carried out at a temperature between 2100- 2300 F.; this temperature is not critical but is made sufficiently high to permit all the required hot-rolling to be carried out with the temperature well above the critical temperature required for the last roll, as will appear hereinafter. Thereafter, on each subsequent roll, the thickness of the strip is reduced by about 250 mils. When the strip thickness is down to about 250-300 mils, the strip is allowed to cool to a predetermined temperature and then the last hot-rolling pass is made.

As will be shown, once the strip is in condition for the last hot-rolling pass, the problem of securing highly embrittled metal with predetermined grain size is greatly influenced by these factors in combination:

(a) The temperature of the metal during the last hotrolling pass,

(b) The degree of reduction during the last hot-rolling pass,

The speed with which the metals temperature is brought to below the recrystallization temperature after the last hot-rolling pass.

Of these several factors, factor (a) is a function of the sulphur content so that sulphur content too is, in a sense, a factor that affects particle size. With respect to factor (0), this factor is a function not only of the temperature of the quenching medium but also of the speed with which the strip passes through the rolls; obviously a higher rolling speed results in a more rapid cooling of the strip since less time elapses before the metal is subjected to quenchmg.

In order that the metal can be embrittled and clean grains obtained with precipitation of sulphides and oxides on the grain boundaries, the final hot-rolling must be performed at a temperature where the recrystallization speed of the alloy itself is faster than the diffusion speed of the sulphur and oxygen back into the lattice structure of the alloy. When this is done, the sulphides tend to move with the recrystallization front since the diffusion of the sulphur back into the grains is very much less than that of the nickel, iron or molybdenum. With respect to Permalloy, this temperature range is from the recrystallization temperature which is about 1250 F. to an upper temperature of approximately 1600 F. In actual practice, the lower portion of this temperature range is not entirely satisfactory since the metal becomes so brittle that it readily cracks, thereby making it quite difiicult to handle because it breaks up into quite small pieces even before the final hot-rolling. Therefore, it is desirable that the last hot-rolling occur at a somewhat higher temperature which is selected in accordance with the sulphur content of the metal; in general, the desired hot-rolling temperature in creases with an increase in sulphur content. This is entirely logical since an increase in sulphur brings about a corresponding increase in the amount of embrittling sulphides, and undue embrittlement can only be overcome by increasing the temperature as this increases the diffusion of the sulphur into the crystal lattice structure with a resultant lowering of the metals brittleness. Numerous experiments have shown that the temperature of the metal just prior to the last reduction preferably should be within one of the following temperature ranges in accordance with sulphur content.

Free sulphur content: Hot-rolling temperature, F.

The temperature range which is selected according to the above table is, in each case, the minimum temperature at which the last reduction can occur and yet not have undue embrittling occur prior to the last roll so that the strip will break up prior to issuing from the rollers. Temperatures appreciably above those indicated in the table are to be avoided since, for higher temperatures, the greater energy of the sulphur atoms tends to increase their back difiusion into the lattice whereby the embrittlement is considerably impaired.

The grain size which results after any deformation occurring above the recrystallization temperature is closely related to the degree of reduction which takes place; the higher the amount of reduction, the smaller the grain size and vice versa. Where the reduction is substantial, a great many nuclei are formed on the last roll and these are quite closely spaced so that only small grains result; but where the degree of reduction is slight, only a relatively small number of nuclei are formed and these are quite distantly spaced so that the grain size which then results is much larger. Therefore, when it is desired that the final product have a quite large grain size in order that a powder with large particles be obtained, the final reduction on the last hot-rolling pass is quite small, in the order of approximately 820%. When an intermediate grain size is desired, the final reduction may be in the order of 20- 30% and, for a very fine grain size, in excess of 40%.

The grain size is also considerably influenced by the rate of cooling of the metal strip after the last hot-rolling pass since the grain size tends to increase after the last pass as long as the metals temperature remains above the recrystallization temperature. If a small grain size is desired, it is then desirable to quench the material as quickly as possible after the strip issues from the rolls. This may be accomplished by rolling at high speed and immersing the hot strip quickly into circulating cold water. On the other hand, a much slower cooling rate occurs if the rolling is carried out at slower speed and the strip is thereafter quenched in water at boiling temperature. A still slower cooling and a correspondingly larger grain size results when the strip is cooled in air.

Since both the degree of reduction on the last hot roll and also the rate of cooling thereafter have a considerable effect on grain size, both these factors must be taken into account when a particular particle size is sought. If only the degree of reduction on the last hot-rolling pass is varied as between different heats but each is subjected to the same cooling conditions, only a relatively narrow range of grain sizes can be produced. By also varying the cooling rate, it is possible to extend the possible range of grain sizes quite appreciably. Thus, at one extreme, slight reduction on the last hot-rolling pass in order of 11% followed by a rather slow cooling as by quenching of the strip in boiling water or air, will result in a quite large grain size and thus a rather coarse powder. This is shown in FIGURE 1 of the accompanying microphotographs which illustrates the crystalline structure of a heat subjected to just such treatment. The other extreme is that which produces the extremely fine grain size of FIGURE 4 where the material is subjected to a quite heavy reduction, in the order of 39%. In order to inhibit grain growth after the last rolling, the strip of FIGURE 4 has been quickly cooled by rolling at high speed, i.e., at one meter per second rather than the one-half meter per second as for the same of FIGURE 2, and by immersing it in cold circulating water whose temperature is approximately 50 F.

FIGURES 2 and 3 illustrate intermediate situations. In FIGURE 2, the Permalloy strip has been given a 12% reduction and thereafter quenched in water with a temperature of 200 F. and it .will be noted that the resulting grain size is substantially smaller than that of FIGURE 1. In FIGURE 3, the material shown .has been given a somewhat greater reduction (26%) on the last hot rolling pass, and has been cooled more rapidly by both increasing the rolling speed to one meter per second and by quenching in water at room temperature.

FIGURE 5 illustrates graphically the results which were obtained experimentally with two different heats. In this graph, the abscissa represents the degree of reduc tion during the last hot rolling pass and is expressed in terms of percentage reduction. The ordinate is expressed in terms of the number of grains per unit area so that an increase in this value represents a decrease in grain size. The upper line is a plot based upon the results obtained with four different degrees of reduction (each represented by a difierent point lying along the line), with all other conditions being maintained exactly the same. This clearly shows the direct and substantially linear relationship between degree of reduction on the final hot-rolling pass and grain size. The lower line represents the results obtained with a similar set of tests on a different heat containing a slightly greater amount of sulphur, i.e., .013% as opposed to .012% for the upper line. Because of this slightly greater sulphur content, the hot rolling for the heat represented by the lower line was carried out at a slightly higher temperature, i.e., 1620 F. rather than 1600 F. This means that a slightly greater time must elapse before recrystallization temperature is reached, and this accounts for the slightly larger grain size of this heat. The lower line of the graph also shows the direct relationship between degree of reduction and grain size.

During the last hot-rolling of the alloy strip, the metal ordinarily crystallizes and becomes so brittle that it breaks into small pieces. These pieces may then be ball-milled or pulverized in any known manner to produce a fine powder.

Even in large quantity, production runs, it has been possible to obtain a close control over the final particle size. For example, by eifecting a reduction on the last hot-rolling pass amounting to 35-50%, rolling at one meter per second, and quenching in ice water, as much as 83% of a fine (-400 mesh) powder has been obtained.

Having described an improved process for treating alloys and particularly Permalloy in order to obtain metallic powders having predetermined particle sizes, we desire it to be understood that our invention is not to be limited to the specific steps disclosed and that various modifications thereof are intended to be included within our invention as defined by the appended claims.

We claim:

1. A process for producing powdered Permalloy comprising the steps of:

(a) adding to a melt of the metal containing a minimum amount of calcium, manganese, or magnesium sufficient sulphur uncombined with calcium, manganese or magnesium to comprise substantially .005 to 20% of the total by weight,

(b) forming an ingot of said metal melt,

(c) hot-rolling the metal at a temperature in excess of 1800 F. which permits recrystallization without accumulation of sulphides on the grain boundaries,

(d) cooling said hot-rolled metal,

(e) hot-rolling said metal again for the final time employing a greater degree of reduction to produce smaller particle sizes and a lower degree of reduction to produce larger particle sizes.

(1) quenching said metal at a quick quenching rate to produce smaller particle sizes and at a slower quenching rate for larger particle sizes,

(g) said cooling step reducing the temperature of said metal to a value at least in excess of that at which the grain boundaries are substantially fully covered by sulphides but being sufiiciently low so that said final hot-rolling and quenching ensure that said grain boundaries upon quenching are substantially fully covered by sulphides,

(h) and grinding the cooled metal to produce said powdered Permalloy.

2. The process of claim 1 in which in order to produce a particle size in the range of about 10-20 microns the degree of reduction effected on step (e) is about at least 35% and the metal is quickly quenched according to step (f) by quenching in cold circulating water, whereas for a particle size of about 40 microns the reduction of step (e) is only in the range of about 8-20% and the metal is quenched thereafter according to step (1) at a much slower rate by quenching in air, and intermediate particle sizes are obtained by varying the degree of reduction and quenching speed between the aforesaid respective limits.

3. The process of claim 1 wherein the final hot-rolling of step (e) is carried out at a temperature of the metal selected in accordance with said uncombined sulphur content of the Permalloy as follows:

Sulphur content: Hot-rolling temp. F.

4. A process for producing a powdered metal alloy, said alloy comprising those metals forming non-malleable sulphides, the steps comprising:

(a) adding to a melt of the metal containing a minimum amount of calcium, manganese, or magnesium sufiicient sulphur uncombined with calcium, manganese or magnesium to comprise substantially .005% to 20% of the total by weight,

(b) forming an ingot of said metal,

(0) hot-rolling the metal at a temperature in excess of 1800 F. which permits recrystallization without accumulation of sulphides on the grain boundaries,

(d) cooling said hot-rolled metal,

(e) hot-rolling said metal again for the final time employing a greater degree of reduction to produce smaller desired particle sizes and a lower degree of reduction to produce larger desired particle sizes,

(1) quenching said metal at a quick quenching rate to produce smaller desired particle sizes and at a slower quenching rate to produce larger particle sizes,

(g) said cooling step reducing the temperature of said metal to a value at least in excess of that at which the grain boundaries are substantially fully covered by sulphides but being sufficiently low so that said final hot-rolling and quenching ensure that said grain boundaries upon quenching are substantially fully covered by sulphides,

(h) and grinding the cooled metal to produce said powdered metal.

5. A process for producing an embrittled metal alloy including at least one non-malleable sulphide of the constituent metals of said alloy, the process comprising a final hot-rolling of the alloy followed by a quenching of the metal, said final hot-rolling taking place at a temperature within a range which is in excess of that at which the grain boundaries are substantially fully covered by sulphides but sufficiently low so that said final hotrolling and quenching to ensure that said grain boundaries upon quenching are substantially fully covered by sulphides, the degree of reduction of the metal effected by the final hot-rolling being of a greater degree for a small grain size and a lower degree for a large grain size, and cooling said metal after said final hot-rolling quickly to produce smaller grain size and slower to produce larger grain size.

6. The process of claim 5 where the metal is Permalloy and for a particle size in the range of 10-20 microns, the degree of reduction is about at least 35% and the metal is quickly cooled Whereas for a particle size of about 40 2,407,862 Wulff Sept. 17, 1946 microns the degree of reduction is in the range of about 2,793,108 Franklin May 21, 1957 only 23-20% and the metal is slowly cooled.

OTHER REFERENCES References Cited in the file of this Patent 5 Goetzel: Treatise on Powder Metallurgy, vol. I, Inter- UNITED STATES PATENTS science Pub. Inc. (1949), pp. 3639. 1, 9, 49 m et 1, 15,192 h Metals Handbook, PP- 1030 and 1034 (1948 1,739,052 White Dec. 10, 1929 Edmon). 2,368,232 wulff J 3 45 Wulfi: Powder Metallurgy, published by American 2 3 1 22 w lfi Aug 7 1945 10 Society of Metals, 1942, pp. 137, 138.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0a 8,141,760 July 21 1964 Guente Bo Finke' et 31.

It is hereby-certified that error appears in the above humbered teten't requiring correction and that the said Letters Patent should read as co'rrectedbelow.

Column 6 line. 65; strike 0ut t0" Signed and sealed this 1st day of December 1964.

ERNEST W. SWI'DER Commissioner of Patents t testing Officer 

1. A PROCESS FOR PRODUCING POWDERED PERMALLOY COMPRISING THE STEPS OF: (A) ADDING TO A MELT OF THE METAL CONTAINING A MINIMUM AMOUNT OF CALCIUM, MANGANESE, OR MAGNESIUM SUFFICIENT SULPHUR UNCOMBINED WITH CALCIUM, MANGANESE OR MAGNESIUM TO COMPRISE SUBSTANTIALLY .005% TO .20% OF THE TOTAL BY WEIGHT, (B) FORMING AN INGOT OF SAID METALY MELT, (C) HOT-ROLLING THE METAL AT A TEMPERATURE IN EXCESS OF 1800*F. WHICH PERMITS RECRYSTALLIZATION WITHOUT, ACCUMULATION OF SULPHIDES ON THE GRAIN BOUNDARIES, (D) COOLING SAID HOT-ROLLED METAL, 